A hardener is required when using a body filler for automotive repair because it helps to cure and set the filler material. Body filler is a type of putty that is used to fill in dents, holes, and other imperfections in a car's bodywork.

The filler material is typically made of a mixture of resin and a filler material, such as fiberglass or talc, and is applied to the damaged area. The hardener, when mixed with the filler material, triggers a chemical reaction that causes the mixture to harden and set.

This hardening process gives the body filler its strength and rigidity, making it suitable for use in automotive repair. The type and amount of hardener used will depend on the specific filler material being used and the conditions under which the repair is being performed.